1. Field of the Invention
The present invention relates to a sensing device, and more particularly relates to a thermal sensing device able to enhance sensing signal output from a measurement circuit.
2. Description of Related Art
A sensing mechanism of a thermal sensing device is that the sensing materials of the thermal sensing device will change material characteristics because of temperature. Therefore, a measurement circuit is used to convert a variation value of the material characteristics of the sensing materials to a signal output. A manufacturing method of the thermal sensing device is to dispose a sensing resistor with temperature coefficient of resistance on a suspending structure made by a semiconducting material with good insulation effect. When current passes through the sensing resistor on the suspending structure, the temperature of the sensing resistor is higher than room temperature because heat is hard to dissipate away. The temperature of the sensing resistor will be varied in accordance with physical variation so as to change the resistant value of the sensing resistor.
With reference to FIG. 12, the conventional thermal sensing device 120 includes a substrate 121, an insulating layer 122, a sensing resistor 123 and a cavity 124. The insulating layer 122 is disposed on a surface of the substrate 121 and the sensing resistor 123 is disposed on the insulating layer 122. The cavity 124 is formed by hollowing an internal space of the substrate 121 of the thermal sensing device 120, and the insulating layer 122 and the sensing resistor 123 form a suspending structure 125 above the cavity 124. Since the suspending structure 125 is connected to the insulating layer 122 on the substrate 121 via a connecting portion 126 of the suspending structure 125, a thermal conductive effect between the sensing resistor 123 and the substrate 121 is reduced. Therefore, the heat generated by the sensing resistor 123 transmitting to the substrate 121 is reduced to improve the sensing effect.
With reference to FIG. 13, the thermal sensing device is usually implemented in a Wheatstone Bridge circuit 130 composed of four resistors to perform a measurement. The Wheatstone Bridge circuit 130 includes a first resistor 131, a second resistor 132, a third resistor and a fourth resistor 134. The resistant value of the sensing element is varied in accordance with the temperature, so a variation of a voltage difference at middle of the Wheatstone Bridge circuit 130 is used to measure physical quantities. As shown in FIG. 13, in the Wheatstone Bridge circuit 130, the thermal sensing device is implemented to be the third resistor 133, but the first resistor 131, the second resistor 132 and the fourth resistor 134 are normal resistors that will not be varied in accordance with the temperature. The resistant values of the first resistor 131, the second resistor 132, the third resistor 133 and the fourth resistor 134 are R1, R2, R3(T) and R4 respectively. Equations of the Wheatstone Bridge circuit 130 are:
            Δ      ⁢                          ⁢      Vs        =                  V        ⁢                                  ⁢        34            -              V        ⁢                                  ⁢        12              ,            V      ⁢                          ⁢      12        =          Vb      ⁡              [                              R            2                                              R              1                        +                          R              2                                      ]              ,            V      ⁢                          ⁢      34        =          Vb      ⁡              [                              R            4                                                              R                3                            ⁡                              (                T                )                                      +                          R              4                                      ]            
V12 is a voltage value at a node between the first resistor 131 and the second resistor 132 and V34 is a voltage value at a node between the third resistor 133 and the fourth resistor 134. Vb is an operating voltage of the Wheatstone Bridge circuit 130. If the third resistor 133 includes positive temperature coefficient of resistance (PTCR), the resistant value R3(T) of the third resistor 133 is proportional to the temperature. When the detected physical value is changed, for example a thermal conductance of the sensing element is increased when air pressure is raised, the temperature of the sensing element is decreased and the resistant value R3(T) of the third resistor 133 is reduced. Therefore, the V34 voltage is increased. Since the V12 voltage is irrelevant to the temperature, a variation value Vs of the output voltage is increased and a value of the detected physical value can be calculated in accordance with the variation value Vs of the output voltage.
With advances in Micro Electromechanical System (MEMS) technology, the size of the sensing element is getting smaller and smaller, so a sensing signal calculated in accordance with the resistant variation of the sensing resistor 123 is not high enough. If the detected sensing signal can be increased, the problem that the signal becomes weaker because of a miniaturization process of the sensing element can be resolved. Therefore, a need arises to design a thermal sensing device to increase the sensing signal outputted from the measurement circuit so as to resolve the aforementioned problem.